This invention broadly relates to a method and an apparatus for tightening fasteners and more particularly to a method and apparatus for tightening nuts on axially connected rod-like members.
It is a well known method to combine two bars in a linear fashion by providing the mating ends of the bars with threads and connecting the bars with at least two nuts adapted to engage the threads in a fashion so that one nut functions as a coupling unit while the other nuts lock the coupling nut against movement. When utilizing this method, some amount of backlash remains between the threads of the bars and the threads of the nuts and if the nuts are not sufficiently tightened, this residual backlash causes an easy loosening of the coupling or a bending at the coupling of the bars. The effect of backlash is of particular concern when two reinforcing iron bars are combined by this method and buried in concrete, because the action of external forces upon the concrete structure often times bends the reinforcing iron bars at the coupling causing resulting cracks in the concrete surfaces above the loosened coupling.
In order to overcome the deleterious effect of backlash, studies have shown that minimum torque of 200 kg-m should be applied to the tightening nuts when coupling bars having a diameter of 32 mm. Since the conventional use of wrenches to tighten nuts by hand is insufficient to apply the requisite torque, often times, automatic tightening devices are used such as the hydraulic wrenches conceived by the present inventor and described in Japanese application No. 123999/76, No. 24277/77, and No. 36800/78.
All of these prior art hydraulic wrenches include two axially aligned spanners attached to a main body, a rack device for rotatably driving one of the spanners having an outer surface provided with cogs, and a hydraulic cylinder for driving the rack back and forth. Since one spanner is rigidly affixed to the main body and the other spanner is driven to rotate with respect to the main body, when the device is attached to two nuts to be tightened, and the hydraulic cylinder is charged, the resulting torque is applied equally between the two spanners. As a consequence, the main body of the device rotates in order to tighten both nuts simultaneously.
Some inconveniences have been noted with respect to rack-type hydraulic wrenches. Because the rack which is adapted to rotate the spanner is driven by a single stroke of the hydraulic cylinder, the length of the rack and the stroke of the hydraulic cylinder must be very long. As a result, devices of the rack-type are excessively large and heavy making it difficult for one operator to lift and situate the device in place.
Further, the rack and cogs formed on the driven spanner cannot be disengaged from one another. Therefore, if obstructions close to the bars to be coupled restrict the ability of the main body to rotate, both nuts cannot be tightened.
Additionally, if the main body of the hydraulic wrench encounters an obstacle during the tightening operation, the wrench cannot be removed without reversing the operation of the hydraulic cylinder thereby loosening the nuts to some extent.